Known in the past has been an internal combustion engine providing an air-fuel ratio sensor in an exhaust passage of the internal combustion engine and controlling an amount of fuel which is fed to the internal combustion engine based on an output of the air-fuel ratio sensor.
The air-fuel ratio sensor used in such an internal combustion engine gradually deteriorates along with use. As such deterioration, for example, deterioration of response of the air-fuel ratio sensor may be mentioned. The deterioration of response of the air-fuel ratio sensor occurs due to air holes provided in a sensor cover for preventing a sensor element from being covered by water ending up being partially clogged by particulate matter (PM). If the air holes are partially clogged in this way, the exchange of gas between the inside and outside of the sensor cover becomes slower, and as a result the output of the air-fuel ratio sensor ends up becoming blunter. If such deterioration of the air-fuel ratio sensor occurs, the various control operations performed by the control system of an internal combustion engine end up being hindered.
Therefore, diagnosis systems diagnosing deterioration of air-fuel ratio sensors have been proposed (for example, see PLTs 1 to 5). As such a diagnosis system, for example, one making a target air-fuel ratio change in a step manner and along with this detecting a first response time until an output value of the air-fuel ratio sensor reaches a first predetermined value and a second response time larger than the first predetermined value and using the two times of the first response time and the second response time as the basis to judge deterioration of the air-fuel ratio sensor has been proposed (for example, PLT 1). Here, as patterns of deterioration of an air-fuel ratio sensor, there is deterioration of response where the response time becomes slower and deterioration of gain where the response itself increases or decreases. As opposed to this, according to the diagnosis system described in PLT 1, by using the first response time and the second response time as the basis to judge deterioration of an air-fuel ratio sensor, it is considered possible to accurately identify by which of the two patterns of deterioration the deterioration of the air-fuel ratio sensor is being caused.